The ability to pump predetermined volumes of liquids at a preset flow rate is important in many fields of industry and particularly in the medical field. Delivery of precise amounts of liquids at precise, preset flow rates is routinely required in hospital and clinical settings. The principal infusion systems in common use today use either gravity flow with flow control devices or pump delivery systems. This invention relates to improvements in pump delivery systems particularly useful as infusion pumps for metering preset volumes of parenteral solutions to patients at preset flow rates with light weight, low power, portable pumping units.